Good question, and I'm not ashamed to admit that I don't know the answer

However, I'd say that it probably doesn't matter what you set this to; I suspect you'll get the same output no matter what value you use. The reason is that, since you're spitting out an I-frame only MPEG2 file, there are no GOPs--each frame is independent and does not reference another frame in the stream. That's what a GOP is--a group of pictures that contains a number of different frame types that can reference other frames within the GOP for encoding efficiency.

I'd suggest doing a quick test: export a 10 second sample with the closed GOP setting at 0, and then at 1 (or some other value). I don't think you'll see any difference, either qualitatively or quantitatively.